An EPDM flashing composition comprises 100 parts by weight of an elastomeric polymer comprising from about 85 to 100 parts by weight of at least one ethylene-propylene-diene terpolymer having up to about 2 percent by weight crystallinity and from 0 to about 15 parts by weight of an ethylene-propylene-diene terpolymer having more than 2 percent by weight crystallinity; from about 40 to 120 parts by weight of a filler selected from the group consisting of reinforcing and non-reinforcing materials and mixtures thereof, per 100 parts of the polymer; from about 40 to 105 parts by weight of a processing material and mixtures thereof, per 100 parts of the polymer; and from about 0.5 to 6 parts by weight of a cure package, per 100 parts of the polymer, the cure package comprising from about 0.1 to 1.25 parts by weight sulfur and from 0.4 to 5 parts by weight of at least one sulfur vulcanizing accelerator and being devoid of hexasulfides and thioureas. The uncured EPDM flashing composition has a longer shelf-life than current uncured, calendered commercial EPDM flashing materials, and meets the flexibility, extensibility and green strength requirements and cure characteristics desired of comer flashing compositions. Preferably, the composition has a Mooney viscosity (ML/1+4 at 135 .degree. C.) of less than about 35 Mooney units and a green strength at break of less than 65 psi.